• Breathtaking autumn colors in Bering Land Bridge National Preserve

    Bering Land Bridge

    National Preserve Alaska

Geologic Formations

Up close view of a hand holding a brunton to a rock, a device used to measure geological features

A brunton is used to measure geological features

NPS Photo - Andrea Willingham

In the study of geology, there are three basic types of rocks: sedimentary, igneous, and metamorphic. All three of these types can be found on the Seward Peninsula, as well as several other geological features unique to the area because of its northern lattitude and arctic climate.

Sedimentary - Through the process of weathering and erosion, little bits of earth are constantly being broken down and worn away; they eventually settle at the bottom of lakes and rivers, and over time turn into sedimentary rock. In these layers of material, known as "sediments," fossils can be pressed in, as well as various kinds of other rocks and minerals. Eventually all of these harden together through pressure or cementing (when minerals dissolve in the water and "glue" everything together). Sedimentary rocks include sandstone, limestone, and shale.

Igneous - Formed originally from liquid magma deep in the earth, igneous rocks are created as they cool and are pushed towards the surface. The speed at which the magma cools can contribute to the type of rock it forms. Igneous rocks that cool slowly (like granite) often contain large crystals. If it cools faster (like basalt), it forms smaller crystals, or it may cool so fast that it contains no crystals at all and looks shiny or glass (like obsidian).

Metamorphic - Originally an igneous or sedimentary rock, transformed (or "morphed") by pressure and heat. The pressure and heat can change its chemical makeup and turn it into a totally different kind of rock! For example, through this process sandstone can become quartzite, shale can become slate, and limestone can become marble, all through a combination of heat and pressure deep down in the earth!

A person in an orange vest holding a fragment of schist rock out towards the foreground

Schist fragment

NPS Photo - Andrea Willingham

On the Seward Peninsula

The majority of the Seward Peninsula is made up of metamorphic blueschist, as well as abundant limestone and marble. Many of the mountains are made up of igneous granite and metamorphic gneiss (pronounced "nice"). Gneiss can be identified by its banded patterns from its formation in the deep, fiery depths of the earth where the rock was nearly hot enough to be liquid. Dark colored igneous basalt can also be found on the Seward Peninsula, derived from volcanoes. In Bering Land Bridge National Preserve, some of the most notable geologic features are found around Serpentine Hot Springs, the Imuruk Lava Flows, and the Devil Mountain Maars. Many of these phenomena are created by the combined interactions permafrost, seasonal freezing and thawing, volcanic activity, and extreme weather conditions of the Seward Peninsula.

A wall of exposed permafrost with grass hanging over the top, and a clump of willows at its base.

Exposed permafrost layer

NPS Photo


Just like it sounds, permafrost is "permanent frost," or permanently frozen ground where the temperature has remained below 32oF continuously for more than two years. Continuous permafrost underlies almost the entire tundra of the Arctic and Northwest Alaska. Its upper layer is known as the permafrost table, which acts as a barrier so that no water can move (percolate) through the ground. It can extend more than 2,000 feet underground to where it gets too warm from heat at the earth's core.

Well that's cool (literally and figuratively!), but why is permafrost important, you ask? As a matter of fact, permafrost is the world's largest "carbon sink." In other words, permafrost stores carbon - actually 1,672 billion metric tons, or twice the amount of carbon stored in the atmosphere! Basically when a plant or animal dies above the permafrost, it does not decay and release its carbon into the atmosphere, but instead eventually freezes into the permafrost.

With warming temperatures from climate change however, the permafrost is melting. This makes the soil weak and unstable, and things like buildings built on top of it can shift or collapse. In addition, the carbon that was stored up inside will be released into the atmosphere, adding to the greenhouse gases already accelerating the rate of climate change. So, long story short, permafrost is a vital part of the Arctic environment.

An aerial view of the green tundra under cloudy skies, with polygon patterns on the ground and two pingos in the distance

Low-center polygons on the tundra, with two pingos in the background

NPS Photo


These formations are created in areas with permafrost and seasonal frost as a result of contraction cracks from wedges of ice. The pressure created by an ice wedge forces the soil upward around the crack to form 2 small ridges. This creates a concave, low-center polygon.

Another type that can form is a high-center polygon, created in poorly-drained areas where water fills the center of the polygon and the ice wedge troughs. The collected water conducts heat from the sun, melting the underlying permafrost, deepening the ice wedge troughs. Eventually these can grow large enough to form a lake.

An aerial view of a large round pingo, with a pond in the distance

A large pingo on the tundra

NPS Photo - Andrea Willingham


The word "pingo" comes from an Inupiaq name for a cone-shaped hill or mound of soil with a core of ice. It can be anywhere from a few feet high to over 200 feet high and 2,000 feet in diameter.

There are two types of pingos: closed system and open system.

Closed-system pingos are the most common type, and form in relatively level areas where unfrozen groundwater is trapped by permafrost. The water is forced inward where it then freezes and expands, heaving the overlying ground upward and creating the mound that you see on the tundra.

Open-system pingos are usually smaller and forms when groundwater flows downhill and gets trapped beneath the permafrost. The liquid water pushes itself up through cracks in the permafrost, where it then freezes and expands, pushing the overlying soil into little cone-shaped mounds.

Pingos can grow by as much as 5 feet per year and then continue growing slowly (about 1 foot per year) for millennia.

Did You Know?